This invention relates to an image processing technology, and in particular, to an image processing technology for use in an electronic camera.
Accompanied by the improvement of the electronic technology, an electronic camera which converts an image sensed into digital data to memorize them has been developed and already on the market. A user can display an image that has been sensed by an electronic camera on the display of his own personal computer for example, and further, can print the image through a printer; therefore, its range of application has become broad.
Incidentally, an image which has been sensed by an electronic camera is converted into image data, which are stored in a memory as compressed. In this case, the image data volume after compression processing (what is called the file size) usually varies in accordance with the photographic object sensed. That is, a low frequency image such as the sky is compressed with its file size made comparatively small, and a high frequency image such as a woods is compressed with its file size made comparatively large. Such compression processing is called compression processing of variable length.
Owing to such a characteristic of compression as mentioned above, assuming that the memory capacity of the memory medium of an electronic camera is constant, there is a problem that it cannot be known until image sensing has been completed how many image frames can be stored in such a memory medium; this is different from a camera for a silver halide film.
For such kind of a problem, compression processing of fixed length has been developed. According to such compression processing of fixed length, for example, in the case where the file size is fixed at 100 KB, because all images are stored as compressed to 100 KB, 80 image frames can be stored in the case where a memory medium of 8 MB is used; therefore, a user of an electronic camera can immediately understand how many image frames are left to be sensed, which is convenient to him.
However, in the compression processing of fixed length, there is a problem that it needs pre-processing in which the frequency component of the image is analyzed beforehand so as to determine the ratio of compression, which makes the processing time increase. For such a problem, it has been also considered that a high ratio of compression is set beforehand in order that an image of any kind may be made always to have a file size of 100 KB or under. However, if the ratio of compression is set at a high value, image quality becomes low in proportion to it; therefore, the actual situation is that compression processing of an excessively high compression ratio should be avoided.